Semiconductor inspection and metrology involves the use of highly sophisticated imaging systems to inspect the surface of a semiconductor wafer or photomasks used in the fabrication of integrated circuits on semiconductor wafers (collectively referred to as “samples” or “specimens” herein) in order to detect defects that may occur during fabrication. Certain advanced imaging systems used for semiconductor defect inspection can detect defects on the order of 30 nm in size during a full inspection of a 300 mm diameter wafer. Such defects are several orders of magnitude smaller than the wafer itself. Advanced imaging systems exhibiting sufficient throughput for wafer inspection systems and photomask inspection systems typically employ Charge-Coupled Device (CCD) sensors and associated TDI drive electronics, and such imaging systems are referred to herein as TDI imaging systems.
FIG. 6 illustrates a generalized CCD sensor utilized in conventional TDI imaging systems. The sensor include a CCD pixel array forming an imaging region 101 in which the pixels are arranged in horizontal rows and vertical columns 102 (e.g., 256×2048 or larger). CCD sensors typically contain channel stops 103, represented by the solid vertical lines in FIG. 6, that prevent the movement of image charges (photoelectrons) from one column to another within the imaging region 101. Image charge movement is thus restricted along columns 102 (i.e., downward in the vertical direction) toward serial registers 104 disposed along the lower edge of the sensor. When an image charge reaches the last pixel in a column, the image charge is transferred by serial registers 104 horizontally, pixel by pixel, until the charge reaches a read-out stage, from which it is transferred to a read-out amplifier or amplifiers 105. A transfer gate 106 or similar structure typically controls charge movement between the imaging region 101 and serial registers 104. Certain sensors have only one read-out amplifier 105, typically positioned at the end of the serial register 104. Other sensors, such as the one shown in FIG. 6, have multiple read-out amplifiers 105 to decrease the time required to read the contents of the pixels in the serial register.
In a typical TDI imaging system arrangement that utilizes a CCD sensor (such as that shown in FIG. 6), a conveying mechanism (not shown) causes the sample to move relative to the sensor while a lamp, laser beam, or other bright illumination light source (not shown) illuminates the sample (e.g., semiconductor wafer) surface. The reflected light is projected/guided onto the sensor, causing the sensor to generate photoelectrons in the pixels that form image charges representing the amount of received reflected light. The sample is scanned such that image charges, which are generated for each small region of the sample's surface, are collected and transferred from pixel to pixel along each pixel column (e.g., downward along vertical columns 102 in FIG. 6) at generally the same rate at which the sample moves relative to the sensor. The image charge portions for each imaged surface region that are collected at each pixel location are integrated (summed) with the image charge generated in previous pixels, and then the “final” image charges are read-out and processed to generate a magnified image of the sample surface using known techniques.
Semiconductor inspection and metrology require very stable, low-noise light sources to detect small defects and/or make very precise measurements of small dimensions of features on a semiconductor wafer specimen. Currently, UV light sources (i.e. light sources with wavelengths 100-400 nm) are used in state-of-the-art semiconductor inspection and metrology systems because UV wavelengths provide adequate sensitivity to defects and dimensions of features produced by current semiconductor processing fabrication techniques. However, as semiconductor fabrication technology produces even smaller device features, next-generation semiconductor inspection and metrology systems must be provided that are able to image and measure features with higher resolution than is capable today. In order to achieve this higher resolution goal, next-generation semiconductor inspection and metrology systems must utilize light sources having wavelengths below 100 nm (e.g., 13.5 nm). Unfortunately, state-of-the-art TDI imaging systems cannot be easily modified to utilize light sources having wavelengths below 100 nm in part because such light exhibits significantly higher energy that would prevent state-of-the-art TDI imaging systems from generating useful imaging data.
What is needed is a TDI imaging system and operating method that facilitates next-generation semiconductor inspection and metrology using light having wavelengths below 100 nm.